Illumination apparatuses having an alignable light source and/or an alignable illuminating optical system are used in many optical instruments in order to fully illuminate a specimen that is to be examined, or a portion thereof. One example thereof is microscopes of all types. These are used, among other purposes, as inspection instruments for the examination of semiconductor wafers. A further example concerns lithography instruments with which a specific pattern is exposed onto a substrate.
Uniform and reproducible illumination of a specimen that is to be examined is very important in many cases in terms of the functionality of such optical instruments. Even slight changes in the location of the light source with respect to the illuminating optical system and the optical axis of the optical system can considerably degrade performance.
In particular when arc lamps are used in combination with extremely high-magnification objectives, conventional illumination apparatuses in many cases can no longer meet present-day requirements. This applies in particular to demanding inspection and measurement tasks in the UV (ultraviolet) and DUV (deep ultraviolet) regions. The reasons for this include inadequate stability and hitherto insufficient or complex alignment capabilities.
External influences such as vibrations and shock can result in a misalignment of the light source relative to the illuminating optical system. As a result of high thermal loading and the effect of strong UV light, changes that negatively affect the intensity of the emitted light over time occur on the internal surfaces of the illumination apparatus housing. Such deviations can usually be alleviated by realigning the light source, the illuminating optical system, or both together. Suitable light sources are, for example, an incandescent lamp, halogen lamp, discharge lamp, or laser light source. The illuminating optical system can be constituted by a reflector, a lens element, a lens system, a diaphragm, or a combination of such components.
Especially in illumination apparatuses that are used in instruments under clean-room conditions, it is moreover essential to take into consideration the problem of contamination of the specimens that are to be examined, which can result from the changes in the surfaces of the lamp housing.